The present invention relates to control systems for direct current electric motors and, more particularly, to an improved control system for regenerative braking of a direct current electric motor.
Direct current (d-c) electric motors are often used in traction vehicle drive applications such as, for example, electric locomotives or transit cars. In such applications motive power is controlled by regulating motor current, typically by means of a control system employing a chopper. The chopper is essentially a controlled switch connected in the energizing circuit of the motor armature so as to meter current to the motor by periodically opening and closing. The ratio of the closed time of the chopper to the sum of the closed time and the open time is the duty factor of the chopper. During the closed period of the chopper, the motor armature windings are connected to a power source through a path of relatively low resistance and current builds toward some peak value. During the open period of the chopper, the motor is disconnected from the power source and armature current, circulating through a free wheeling diode, decays from the magnitude attained during the chopper closed time. In this manner, pulses of current are periodically applied to the motor and an average motor current is established. The average motor current tends to remain relatively constant due to the smoothing action of the circuit inductance. In general, the circuit inductance is sufficient to smooth the pulsating current and prevent jerking or lurching of the vehicle so long as the current pulses are supplied at relatively frequent repetition rates, such as for example, 200 to 400 Hz.
An advantage of the d-c electric motor for traction vehicle drives is that the motor may be operated in an electrical retarding or braking mode, when it is desired to decelerate or stop the vehicle, by simply reversing either the direction of field current or the direction of armature current. Generally this reversal is achieved by means of electromechanical contactors, although some recently developed systems have utilized static switching elements. Assuming that the motor has been propelling the vehicle so that an appreciable initial velocity of the vehicle has been achieved, reversal of the field current or reversal of the armature polarity will result in a reversal of power flow and the motor will operate as a d-c generator converting the kinetic energy of the vehicle into electrical energy.
Two types of electrical braking are commonly employed in electrically driven vehicles. These two types are dynamic braking and regenerative braking. In dynamic braking the electrical energy generated by the d-c motor is dissipated in braking resistors which convert it to thermal energy. In regenerative braking the electrical energy is returned to the power source. In vehicles such as electric locomotives or transit cars where electrical power is supplied from an external source, regenerative braking is limited by the receptivity of the external source. For example, rail gaps frequently occur and result in an open circuit between the vehicle and the power source. For this reason, many systems using electrical braking employ a combination of dynamic and regenerative braking and include a control system for blending the two types of electrical braking. Such a blending system is shown, for example, in U.S. Pat. Nos. 3,876,920 and 3,657,625. As illustrated in these patents, the chopper is connected in a shunt arrangement with the motor armature during braking and is used primarily to step-up armature current during low-speed braking.
The chopper control system of these prior art systems operates in the braking mode in a manner similar to the operation in the propulsion mode, i.e., braking torque is regulated by using the chopper to control the average armature current. In a typical traction vehicle system the electrical braking power required may be two to three times the motoring power resulting in the armature generated voltage being two or three times the magnitude of the source voltage. Under this condition it is clear that the motor armature cannot be connected directly to the voltage source during regenerative braking since the difference in potential would result in excessive currents flowing from the motor to the source. Such currents could result in permanent damage to the motor armature if "flashing" were to occur. Accordingly, series resistors are commonly inserted in the motor current path during regenerative braking. However, the series resistors dissipate the regenerative energy and prevent full energy recapture.
In a traction vehicle system using plural motors wherein the motors are arranged to be connected in series across a power source during motoring, an alternative arrangement is to reconnect the motors into a parallel configuration during braking. This arrangment effectively reduces the voltage reflected to the power source but creates an additional current handling problem since the regenerative current is increased by the same factor as by which the voltage is decreased. This necessitates a chopper size increase in proportion to the increase in current. The required increase in chopper size makes this arrangment economically unattractive.
In a system having high regenerative capability, the receptivity of the power source is of great concern. A preferred method for guaranteeing electric braking is to incorporate dynamic braking capability in the vehicle and to provide control means to effect electrical brake blending. However, because the power conditioning means in a highly regenerative system is connected in both a series and parallel configuration during electrical braking, the blending circuits of the prior art do not adequately function in the present system.